1974 Merc 1500

my 1973 18' sea ray has a 1973 merc 850 on it currently, but doesn't move it along as fast as i'd like. so i've been keeping my eyes open locally for a 1350 or 1500 straight 6 from the same era. well i found a 1974 1500 with power trim that is complete with controls and is currently on a 16' glastron. the boat is shot. really no value to it. the current owner got it from an estate sale and knows nothing about it other than it spun over when he hooked up a battery. it was last registered in 2003.

so before checking this motor out, i wanted to get some opinions on what to look for. what should the compression in each cylinder be? should i hold the carbs wide open when checking? i'm bringing my ears and will attempt to start it after checking the compression. the lower end took a pretty good hit and the bottom fin is shot. will the lower unit from my 850 fit this motor? i've read here that it would, but doesn't that also depend on whether or not it is a short or long shaft? and how do i tell short vs long shaft? where should i measure to and from and what should the measurements be?

any other things to look out for? are there any peculiar attributes about a '74 1500 that would make me want to stay away from this model? also, if everything checks out, whats a fair price to pay for it?

any info is greatly appreciated. thanks.

Re: 1974 Merc 1500

Compression should be over a 100 and within 10% of each cylinder. All I can help you with.

Re: 1974 Merc 1500

linckeil said:

my 1973 18' sea ray has a 1973 merc 850 on it currently, but doesn't move it along as fast as i'd like. so i've been keeping my eyes open locally for a 1350 or 1500 straight 6 from the same era. well i found a 1974 1500 with power trim that is complete with controls and is currently on a 16' glastron. the boat is shot. really no value to it. the current owner got it from an estate sale and knows nothing about it other than it spun over when he hooked up a battery. it was last registered in 2003.

so before checking this motor out, i wanted to get some opinions on what to look for. what should the compression in each cylinder be? should i hold the carbs wide open when checking?

Don't Know good question. try one reading closed and one open.

i'm bringing my ears and will attempt to start it after checking the compression. the lower end took a pretty good hit and the bottom fin is shot. will the lower unit from my 850 fit this motor?

Yes, (likely but the gear ratio may be different) most of the I6s were 2:1, many of the four cylinder 80 and 85s were 2.3:1 which you can compensate with a different prop.

i've read here that it would, but doesn't that also depend on whether or not it is a short or long shaft?

Oh yes!!

and how do i tell short vs long shaft? where should i measure to and from and what should the measurements be?

From the bottom of the inside of the motor bracket where it clamps to the transom to the anti cavitation plate. 15" = short shaft 20" = long shaft

any other things to look out for? are there any peculiar attributes about a '74 1500 that would make me want to stay away from this model?

Not that I know of. They should be retarded from the factory timing specs and you should run fresh mid grade fuel. I rebuilt a 1976 1500 and put the 140 pistons in as the 1500 pistons are a little hot for todays terrible fuels.

also, if everything checks out, whats a fair price to pay for it?

Depends on how it checks out. The power trim, (if in good shape) is worth something. In the Seattle area in the dead of winter $500 + or -, now: $800-$1,000. BF Egypt: $1500 - $2000 (if it runs good).

any info is greatly appreciated. thanks.

Click to expand...

Hope I helped ya. Good luck. JR

Re: 1974 Merc 1500

good info, that helps a lot. thank you. so the 1500 needs a high octane to run properly? does it have a high compression ratio necesitating high octane? will there be detonation or any other problem if i ran 87 octane?

so the 140 pistons are more of a dished as opposed to dome design, thus lowering the compression ratio? is that true? what about 140 vs 150 heads? are the combustion chambers a different size between the two?

Re: 1974 Merc 1500

linckeil said:

good info, that helps a lot. thank you. so the 1500 needs a high octane to run properly?

It needs 89 which is mid grade: read my thread about a 1969 Merc 1000 where the big boys weigh in with great info.

does it have a high compression ratio necesitating high octane?

Not so much pure compression ratio as operating temps under load me thinks.

will there be detonation or any other problem if i ran 87 octane?

See the thread I mentioned.

so the 140 pistons are more of a dished as opposed to dome design, thus lowering the compression ratio? is that true?

Not sure about the differences, as I did not do the rebuild. The ace I had do it used the 140 pistons for some reason.

what about 140 vs 150 heads?

Don't have cylinder heads.

are the combustion chambers a different size between the two?

Click to expand...

I don't really know. A big gun: like Clams will have to tell ya the deal.

Bottom line: a 1500 that runs good and has even compression is a great find. If the price and condition is right: buy it. The power trim that I just rebuilt cost me $353. before tax so check that out to make sure it works properly too. Nothing is cheap. I have a bunch of parts motors to keep my old girls runnin'. That said: I luv 'em, n' if the motor yer lookin' at was here under me nose in good condition at the right price I would be unable to resist buyin' it. JR

Re: 1974 Merc 1500

The early 1500's run "hi dome" pistons, very large intake/exhaust ports, and a "power port" hole in the sides of the pistons with corresponding slot in the block for each "hol-ey" piston. Thus moving a lot of fuel/air and making lots of HP at the higher end of the spectrum. The 140 pistons mentioned probably had low domes.

Later inlines got the "low dome" pistons which lowered compression a tad for the newer "wimpy" fuels (same as what OMC did with their V4's and V6's with the "low compression", thicker head gaskets).

AFAIK there's no diff in the domes of the combustion chambers; differences in power on these come from porting, pistons, reeds and carbs.

I'd expect that if you ran the 1500 at 18 deg or so max advance, used midgrade non-alcohol fuel (if available in your area), and propped it for WOT of around 5800 (one person in boat), it'll be quite happy.

By all means do open up the carbs fully when you do compression checks, same as you'd do for a car engine.

Be sure to remove all spark plugs when doing compression check. You can turn the motor over with the ign off by jumping the yellow wire on the solenoid to +12V. Just watch the sparks! Or use an automotive remote starting switch.

Don't ever spin the motor over with the ign key with all the spark plugs removed, unless you reinsert each spark plug back in a plug wire and ground the plugs. It's unlikely you could damage the ign by firing a plug wire without a load on it, but no sense in tempting fate!

Just watch out for the revolving prop since you're going to have the control lever in Fwd gear with full throttle. All the more reason to remove all plugs.

I'd feel very confident in a motor that had 5 psi or less variance between cylinders. 10-15 psi diff might mean carbon buildup which could be fixed with a Seafoam cleaning treatment, but you're always taking a risk of a terminally stuck ring, scuffed piston or scored cylinder.

Best way to check the motor after a compression test is out on the water, but it doesn't sound like the boat it's on will be capable of that.

If you can fire it off, check for water flow. If the exhaust manifold gets quite warm right away, shut it off. If it stays cool or just lukewarm, you've got water flowing thru the powerhead. The water exiting the prop should feel warmer than the supply side, this will also tell you water is circulating thru the motor.

Run it for a few minutes, see if it'll idle down smoothly. It should sound "crisp" and have quite an exhaust "bark". These are not a "doggy" motor so if it sounds lethargic there's probably something wrong with it. Try a real quick, short burst of throttle but don't rev above 2000 or hold it for any length of time (it's hard on the pistons, crank and rods). It should be very responsive, almost immediately accelerating.

If it's not doing those things, it's probably still worth getting (as stated in previous posts, the parts alone, especially the PT&T, certainly have value).

Just factor condition in the price negotiations.

Anyway, back in the Day when we were starting out boating, my buddy and I would have Killed for a 1500!!! I had to 'settle' for a $25 M700 Dockbuster and was happy to get it! A good ol' motor she was, arrr.......she ran sweet on the old Sea King!

Let us know how it turns out..........ed

Re: 1974 Merc 1500

Agree with the above poster... exceptions. Max advance is 21 degrees - no more - no less. Run mid grade fuel. Use a couple cans of mercury power-tune (decarb) as directed on can. Put a new waterpump impeller in it.
Prop it for the high side of 5500 WOT. Run it normally at 4200.

-W